m is substituted for the final m of idam in the Nominative 1st-Case singular.,
As इयम्, अयम् ॥ The substitution of म् for म् is to prevent the अ substitution of 7.2.102. The case-ending is elided by 6.1.68, and य substituted by 7.2.110.,
इदमो मः
Adhyāya 7 · Pāda 2 · Rule 108